Switching power supply

ABSTRACT

A switching power-supply unit uses a time when a transformer voltage Vt inverts due to a rectifier diode (Ds 1 ) entering a non-conducting state as a trigger, and a first switching control circuit (CNT 1 ) turns on a first switching element (Q 1 ) after a predetermined delay time passes. A second switching control circuit (CNT 2 ) turns on a second switching element (Q 2 ) using a time when the transformer voltage Vt inverts due to turning off of the first switching element (Q 1 ) as a trigger. A third switching control circuit (CNT 3 ) turns on a third switching element (Q 3 ) using turning off of the second switching element (Q 2 ) as a trigger. The first switching control circuit (CNT 1 ) determines a period ton 1  of the first switching element (Q 1 ) such that a first output voltage Vo 1  is set to a predetermined value. The second switching control circuit (CNT 2 ) determines an ON-period ton 2  of the second switching element (Q 2 ) such that a second output voltage Vo 2  is set to a predetermined value. The third switching control circuit (CNT 3 ) determines an ON-period ton 3  of the third switching element (Q 3 ) such that a third output voltage Vo 3  is set to a predetermined value.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to methods for controlling a switchingpower supply including a plurality of switching elements and, moreparticularly, to control methods that do not require an oscillationcircuit.

2. Description of the Related Art

Control procedures called PWM (Pulse Wide Modulation) systems and PFM(Pulse Frequency Modulation) systems are generally available as methodsfor controlling switching elements in switching power supplies (SeeNon-Patent Document 1).

PWM systems are systems for controlling the ratio of an ON-period of aswitching element to a switching period, and generally, the switchingperiod is constant. In a case where a plurality of switching elements isprovided, the ON-period ratios of the switching elements are equal toeach other or inverted with respect to each other.

PFM systems are systems for controlling a switching frequency, andgenerally, an ON-period ratio of a switching element is constant. In acase where a plurality of switching elements is provided, therelationships between the ON-period ratios and the switching frequenciesof the switching elements are equal to each other. Non-Patent Document1: Electrical Engineering Handbook (Sixth Edition) by The Institute ofElectrical Engineers of Japan, Feb. 20, 2001, Vol. 20, Chapter 9,Section 2 Switching Regulator, pp 851-852.

In known technologies, when a plurality of switching elements isprovided, an oscillation circuit is also provided. A plurality ofdriving signals is produced based on an oscillation signal of theoscillation circuit, and the driving signals are transmitted to controlterminals of the switching elements. Thus, if a delay time or anadvanced time is generated in a path for transmitting the drivingsignals or a driving circuit, a phenomenon in which the plurality ofswitching elements is in an ON-state at the same time occurs even thoughthe plurality of switching elements connected in series with each othermust be driven in order. This phenomenon may not only prevent normaloperation but may also destroy a power-supply unit due to an overcurrentor the like, thus significantly reducing reliability.

Thus, in order to avoid the phenomenon of the plurality of switchingelements being in the ON-state at the same time, a dead time in whichthe plurality of switching elements is in an OFF-state at the same timeis provided. However, since the dead time does not contribute to voltageconversion, providing a dead time having an unnecessary long duration isone factor that reduces the power conversion efficiency. In addition,since the ON-period ratio and the switching frequency change in PWMsystems and PFM systems, respectively, it is very difficult to set thedead time properly, and a complicated configuration is required.

In addition, in known technologies, obviously, an oscillation circuitfunctioning as a standard is required.

Furthermore, in known technologies, a control process to stabilize anoutput voltage by changing an ON-period of a switching elementfunctioning as a standard is performed. For example, keeping an outputvoltage constant is only one condition for the control.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, preferred embodimentsof the present invention solve the problems caused by a plurality ofswitching elements being in an ON-state at the same time, are capable ofsetting a plurality of conditions for control to satisfy predeterminedconditions, and provide a switching power-supply unit that does notrequire an oscillation circuit functioning as a standard.

A switching power-supply unit according to a preferred embodiment of thepresent invention includes an inductor or a transformer and a pluralityof switching elements switching a current flowing in the inductor or thetransformer and converting power by turning on and off the switchingelements, and including a switching control circuit that turns on thenext switching element in accordance with a change of a voltage or acurrent generated by turning off of a switching element in an ON-state,that sequentially turns on and off the switching elements in associationwith each other, that repeats a series of on-off operations of theswitching elements periodically, that determines an ON-period of each ofthe switching elements in accordance with a condition independentlyprovided for each of the switching elements, and that controls theON-period of each of the switching elements.

A dead time in which two consecutive switching elements from among theplurality of switching elements are turned off is preferably providedbetween ON-periods of the two switching elements and the dead time ispreferably arranged in accordance with a delay time from turning off ofthe switching element in the ON-state and turning on of the nextswitching element.

The dead time is preferably set such that the switching element isturned on when a voltage across the switching element becomes zero or isreduced to near zero.

The next switching element is turned on preferably using a voltage atthe inductor or the transformer generated due to turning off of theswitching element in the ON-state from among the plurality of switchingelements.

The switching control circuit preferably detects an output voltage to aload to determine the ON-period in accordance with the output voltage.

The switching control circuit preferably detects a change or a polarityof a voltage generated at the inductor or the transformer to determinethe ON-period.

The switching control circuit preferably detects the current flowing inthe inductor or the transformer to determine the ON-period.

The switching control circuit preferably detects a voltage across theswitching element to determine the ON-period.

The switching control circuit preferably detects a current flowing inthe switching element to determine the ON-period.

The switching control circuit preferably determines the ON-period of theswitching element such that the switching element is turned off when thecurrent flowing in the switching element becomes zero or reaches nearzero.

According to preferred embodiments of the present invention, sinceturning off of a switching element in an ON-sate causes the nextswitching element to be turned on, the inconvenience of two switchingelements being in an ON-state at the same time basically does not occur,thus improving the reliability of a switching power-supply unit.

In addition, in known technologies, control to stabilize an output bychanging an ON-period of a switching element functioning as a standardis performed, and only one condition is provided for controlling theoutput voltage. In preferred embodiments of the present invention,however, two or more conditions, that is, at most, conditions whosenumber corresponds to the number of switching elements, can beestablished.

In addition, the switching frequency is determined in accordance withthe accumulation of ON-pulses of a switching element, and an ON-periodof each switching element is set. Thus, an oscillation circuit is notrequired.

According to preferred embodiments of the present invention, a dead timebased on a delay time of turning on and off of switching elements isarranged between ON-periods of two consecutive switching elements fromamong the plurality of switching elements, and the reliability of theswitching power-supply unit due to the plurality of switching elementsturning on at the same time is improved. In addition, since the deadtime is set in accordance with the delay time before turning on, thedead time can be properly set easily and the dead time is notunnecessarily increased or decreased even if the switching frequency andthe ON-period ratio changes due to a change in the ON-period of eachswitching element. Thus, the power conversion efficiency can be kepthigh.

According to preferred embodiments of the present invention, since theswitching element is turned on when the voltage across the switchingelement becomes zero or is reduced to near zero, the switching loss canbe significantly reduced and high efficiency can be achieved due to azero-voltage switching operation of turning on at zero voltage.

According to preferred embodiments of the present invention, since theswitching control circuit turning on the next switching element usingthe voltage at the inductor or the transformer generated due to turningoff of a switching element in an ON-state from among the plurality ofswitching elements is provided, a voltage signal generated at theinductor or the transformer can be easily captured as a trigger signaland the switching element can be used as a driving voltage. Thus, asimple circuit structure can be achieved.

According to preferred embodiments of the present invention, since anoutput voltage to a load is detected to determine the ON-period inaccordance with the voltage, a constant-voltage power-supply unit can beeasily configured.

According to preferred embodiments of the present invention, since achange (falling and rising) or the polarity of the voltage generated atthe transformer is detected to determine the ON-period of the switchingelement, a voltage signal generated at the transformer can be easilyused as a trigger signal. Thus, a simple circuit structure can beachieved.

According to preferred embodiments of the present invention, since thecurrent flowing in the transformer is detected to determine theON-period, for example, a conduction period of a rectifier diode and theON-period of the switching element can be set equal to each other. Thus,an effective current and the peak value of a current flowing in therectifier diode and the transformer can be reduced, and the conductionloss is thus reduced.

According to preferred embodiments of the present invention, since thevoltage across the switching element is detected to determine theON-period, the ON-state and the OFF-state of the switching element canbe accurately determined, and this can be easily used as a triggersignal.

According to preferred embodiments of the present invention, since theswitching control circuit detects the current flowing in the switchingelement to determine the ON-period, the state of the switching elementcan be accurately determined and the switching element can becontrolled. Thus, a necessary and appropriate dead time can be provided.

According to preferred embodiments of the present invention, since theswitching control circuit turns off the switching element when thecurrent flowing in the switching element becomes zero or reaches nearzero, the switching loss can be significantly reduced due to azero-current switching operation of turning on at zero current. Thus,high efficiency can be achieved.

Other features, elements, steps, characteristics and advantages of thepresent invention will become more apparent from the following detaileddescription of preferred embodiments of the present invention withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 includes a circuit diagram and a waveform chart of a switchingpower-supply unit according to a first preferred embodiment of thepresent invention.

FIG. 2 includes a circuit diagram and a waveform chart of a switchingpower-supply unit according to a second preferred embodiment of thepresent invention.

FIG. 3 includes a circuit diagram and a waveform chart of a switchingpower-supply unit according to a third preferred embodiment of thepresent invention.

FIG. 4 includes a circuit diagram and a waveform chart of a switchingpower-supply unit according to a fourth preferred embodiment of thepresent invention.

FIG. 5 includes a circuit diagram and a waveform chart of a switchingpower-supply unit according to a fifth preferred embodiment of thepresent invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A switching power-supply unit according to a first preferred embodimentwill be described with reference to FIG. 1. Part (A) of FIG. 1 is acircuit diagram of the switching power-supply unit, and part (B) is anillustration showing the relationship between the waveform of eachsection of the switching power-supply unit and time.

In part (A) of FIG. 1, Vi denotes an input power supply, and T denotes atransformer. A first switching element Q1 is connected to a primarywinding Lp of the transformer T. For a secondary winding Ls of thetransformer T, a first rectification and smoothing circuit constitutedby a rectifier diode Ds1 and a smoothing capacitor C1 is provided. Also,a second rectification and smoothing circuit constituted by a rectifierdiode Ds2, a second switching element Q2, and a second smoothingcapacitor C2 is provided. In addition, a third rectification andsmoothing circuit constituted by a rectifier diode Ds3, a thirdswitching element Q3, and a third smoothing capacitor C3 is provided.

A first switching control circuit CNT1 on/off-controls the firstswitching element Q1, a second switching control circuit CNT2on/off-controls the second switching element Q2, and a third switchingcontrol circuit CNT3 on/off-controls the third switching element Q3. Inthe figure, a broken line entering each of the switching controlcircuits CNT1, CNT2, and CNT3 schematically indicates a trigger path,and a solid line entering each of the switching control circuits CNT1,CNT2, and CNT3 schematically indicates a feedback path.

The first switching control circuit CNT1 receives a voltage (transformervoltage Vt) of the transformer T as a trigger, and turns on Q1 at a timewhen the drain voltage of Q1 falls. The first switching control circuitCNT1 also detects an output voltage Vo1 of a first output terminal OUT1,and determines an ON-period of the first switching element Q1 such thatVo1 is set to a predetermined voltage. In other words, the firstswitching control circuit CNT1 turns off Q1 at a time when the requiredON-period of Q1 has passed.

The second switching control circuit CNT2 receives the voltage(transformer voltage) Vt of the transformer T as a trigger, and turns onthe second switching element Q2 at a time when the voltage (transformervoltage Vt) of the transformer T inverts. Then, the second switchingcontrol circuit CNT2 detects a voltage Vo2 of a second output terminalOUT2, and determines an ON-period of the second switching element Q2such that Vo2 is set to a predetermined voltage. In other words, thesecond switching control circuit CNT2 turns off Q2 at a time when therequired ON-period of Q2 has passed.

The third switching control circuit CNT3 receives a drain voltage of thesecond switching element Q2 as a trigger, and turns on Q3 at a time whenthe drain voltage of Q2 rises. Then, the third switching control circuitCNT3 detects a voltage Vo3 of a third output terminal OUT 3, anddetermines an ON-period of the third switching element Q3 such that Vo3is set to a predetermined voltage. In other words, the third switchingcontrol circuit CNT3 turns off Q3 at a time when the required ON-periodof Q3 has passed.

In part (B) of FIG. 1, Vt represents the voltage (transformer voltage)of the transformer T, and Q1, Q2, Q3, and Ds represent the states of thefirst to third switching elements Q1 to Q3 and the first rectifier diodeDs1, respectively. Here, a high level represents an ON-state, and a lowlevel represents an OFF-state.

(1) State 1 [t0-t1]

First, when the voltage (transformer voltage Vt) of the transformer Tinverts at time t0, after the passage of a delay time Δtd1 from the timet0, the gate voltage of the first switching element Q1 is turned to thehigh-level by the first switching control circuit CNT1, and Q1 is thusturned on. The delay time Δtd1 is set in accordance with a resonanceperiod determined based on the primary inductance of the transformer T,the parasitic capacitance between the drain and source of Q1, and thelike, such that Q1 is turned on at a time when the voltage between thedrain and source of Q1 becomes zero. Thus, a zero-voltage switchingoperation of Q1 is performed, and the switching loss is significantlyreduced.

Then, the first switching control circuit CNT1 determines an ON-periodton1 of Q1 such that the voltage of the voltage Vo1 of the first outputterminal OUT1 is set to a predetermined value. In other words, at timet1, that is, after the passage of Δtd1+ton1 from time t0, the firstswitching control circuit CNT1 turns the gate voltage of Q1 to thelow-level. Thus, Q1 is turned off. The exciting energy of thetransformer T is determined in accordance with the ON-period ton1 of Q1.As a result, the voltage of Vo1 is determined.

(2) State 2 [t1-t2]

When Q1 is turned off, the transformer voltage Vt inverts. The secondswitching control circuit CNT2 receives the voltage of the secondarywinding Ls of the transformer T as a trigger signal, and turns the gatevoltage of the second switching element Q2 to the high-level at time t1when the transformer voltage Vt inverts. Thus, Q2 is turned on after thepassage of a delay time Δtd2 from time t1. The delay time Δtd2 is set inaccordance with a resonance period determined based on the secondaryinductance of the transformer T, the parasitic capacitance between thedrain and source of Q2, and the like, such that Q2 is turned on at atime when the voltage between the drain and source of Q2 becomes zero.Thus, a zero-voltage switching operation of Q2 is performed.

The second switching control circuit CNT 2 determines an ON-period ton2of Q2 such that the voltage of the voltage Vo2 of the second outputterminal OUT2 is set to a predetermined value. In other words, at timet2, that is, after the passage of Δtd2+ton2 from time t1, the secondswitching control circuit CNT2 turns the gate voltage of Q2 to thelow-level.

(3) State 3 [t2-t3]

Since the third switching control circuit CNT3 receives the drainvoltage of Q2 as a trigger signal, when Q2 is turned off at t2, thethird switching element Q3 is turned on after the passage of a delaytime Δtd3 from t2. The delay time Δtd3 is set in accordance with aresonance period determined based on the secondary inductance of thetransformer T, the parasitic capacitance between the drain and source ofQ3, and the like, such that Q3 is turned on at a time when the voltagebetween the drain and source of Q3 becomes zero. Thus, a zero-voltageswitching operation of Q3 is performed.

The third switching control circuit CNT3 determines an ON-period ton3 ofQ3 such that the voltage of the voltage Vo3 of the third output terminalOUT3 is set to a predetermined value. In other words, at time t3, thatis, after the passage of Δtd3+ton3 from time t2, the third switchingcontrol circuit CNT 3 turns the gate voltage of Q2 to the low-level.

(4) State 4 [t3-t0]

When Q3 is turned off, after the passage of a delay time Δtd4 from thetime when Q3 is turned off, the first rectifier diode Ds1 is turned on.This is because Ds1 is turned on due to application of a forward voltageto Ds1 when both Q2 and Q3 are in the OFF-state with the relationshipVo1>Vo3>Vo2.

Then, the first switching control circuit CNT1 determines an ON-periodtond of the rectifier diode Ds1 such that the voltage of the voltage Vo1of the first output terminal OUT1 is set to a predetermined value. Whenthe current of Ds1 becomes zero and an inverse current is applied, thevoltage of the transformer inverts at time t0. In other words, Ds1 isturned off at a time after the passage of Δtd4+tond from time t3, andthe first switching control circuit CNT1 turns the gate voltage of thefirst switching element Q1 to the high-level after the passage of thedelay time Δtd1 from time t0. Time T0 here is equal to the initial timet0.

As described above, by repeating a cycle T represented in part (B) ofFIG. 1 as one cycle, predetermined voltages Vo1, Vo2, and Vo3 can beobtained at the first to third output terminals OUT1 to OUT3,respectively.

With this structure, the next switching element is turned off inassociation with one switching element in an ON-state being turned off.Thus, in other words, an on/off state of each switching element changesin the order of the lapse of time based on causality. Since a delay timeis inevitably required between turning off of one switching element inan ON-state and turning on of the next switching element, this delaytime constitutes a dead time. Thus, the inconvenience of two switchingelements being in the ON-state at the same time basically does notoccur, thus improving the reliability of a switching power-supply unit.Furthermore, properly setting the dead time enables a zero-voltageswitching operation and the like, and power conversion efficiency can bekept high without providing an unnecessarily long dead time.

In addition, since the accumulation of ON-pulses of a switching elementfunctions as a switching frequency, an oscillation circuit is notrequired. Furthermore, voltages of a plurality of outputs (three outputsin the first embodiment) whose number is equal to the number ofswitching elements can be stabilized independently. Although settingvoltages of a plurality of output terminals to respective predeterminedvalues is provided as conditions in this example, a current or the like,instead of a voltage, may be controlled as long as it can be controlledby an ON-period of a switching element. In other words, independentconditions whose number corresponds to the number of switching elementscan be met.

Although turning off of the first and second switching elements Q1 andQ2 is detected based on the drain voltages of Q2 and Q3 in the exampledescribed above, turning off of the switching elements may be detectedby detecting currents flowing in the switching elements. In addition,although the transformer voltage is detected based on the voltage of thesecondary winding Ls of the transformer T as a trigger of Q2 in theexample described above, a change in the transformer voltage may bedetected based on the voltage of the primary winding Lp. Furthermore,instead of detecting falling of the transformer voltage, a change in thepolarity of the transformer voltage may be detected.

In addition, although an operation of setting an output voltage to apredetermined value in the steady state is described in the exampledescribed above, for example, by setting the maximum ON-period of eachswitching element at a transition time until an output voltage reaches apredetermined value, such as a starting time, a series of switchingoperations are repeated periodically, and a steady state is reached.

A switching power-supply unit according to a second preferred embodimentwill be described with reference to FIG. 2. Part (A) of FIG. 2 is acircuit diagram of the switching power-supply unit, and part (B) is anillustration showing the relationship between the waveform of eachsection of the switching power-supply unit and time.

In part (A) of FIG. 2, an inductor Lr is connected to the primarywinding Lp of the transformer T. The second switching element Q2 and acapacitor Cr are arranged so as to define a closed loop together withthe inductor Lr and the primary winding Lp. A rectification andsmoothing circuit constituted by a rectifier diode Ds and a smoothingcapacitor Co is connected to the secondary winding Ls of the transformerT.

The first switching control circuit CNT1 on/off-controls the firstswitching element Q1, and the second switching control circuit CNT2on/off-controls the second switching element Q2. In the figure, a brokenline entering each of the switching control circuits CNT1 and CNT2schematically indicates a trigger path, and a solid line entering eachof the switching control circuits CNT1 and CNT2 schematically indicatesa feedback path.

The first switching control circuit CNT1 receives an inverting time whenthe voltage (transformer voltage) of the transformer T rises as atrigger. The first switching control circuit CNT1 also detects an outputvoltage Vo, and controls the ON-period of the first switching element Q1such that Vo is set to a predetermined voltage.

The second switching control circuit CNT2 receives an inverting timewhen the transformer voltage of the transformer T falls as a trigger.The second switching control circuit CNT2 also detects a voltage vcacross the capacitor Cr, and controls the ON-period of Q2 such that vcis set to a predetermined voltage or vc does not exceed thepredetermined voltage.

In part (B) of FIG. 2, Vt represents the waveform of the transformervoltage, and Q1 and Q2 represent the states of the first and secondswitching elements Q1 and Q2, respectively. Here, a high-levelrepresents an ON-state, and a low-level represents an OFF-state.

(1) State 1 [t0-t1]

The first switching control circuit CNT1 receives a trigger signal attime t0, and after the passage of a predetermined delay time Δt1, thefirst switching control circuit CNT1 turns the gate voltage of Q1 to thehigh-level. Thus, Q1 is turned on. Since the output voltage Vo changesin accordance with the ON-period ton1 of the first switching element Q1,ton1 is determined such that a predetermined output voltage Vo can beobtained. In other words, at a time after the passage of Δt1+ton1 fromtime t0, the gate voltage of the first switching element Q1 is turned tothe low-level, and Q1 is thus turned off.

(2) State 2 [t1-t0]

When Q1 is turned off, the transformer voltage Vt inverts. After thepassage of a delay time Δt2, the second switching control circuit CNT2turns the gate voltage of Q2 to the high-level using the inverting timeof the transformer voltage Vt as a trigger. Thus, the second switchingelement Q2 is turned on.

Since the voltage vc across the capacitor Cr changes in accordance withthe ON-period ton2 of Q2, ton2 is determined such that vc is set to apredetermined voltage. In other words, at a time after the passage ofΔt2+ton2 from time t1, the second switching control circuit CNT2 turnsthe gate voltage of Q2 to the low-level. Thus, Q2 is turned off.

Since the transformer voltage Vt re-inverts when Q2 is turned off, afterthe passage of the delay time Δt1 from time t0, the first switchingcontrol circuit CNT1 turns the gate voltage of the first switchingelement Q1 to the high-level using the re-inversion of the transformervoltage Vt as a trigger. Time t0 here is equal to the initial time t0.

As described above, by repeating a cycle T shown in part (B) of FIG. 2as one cycle, operation as a voltage-clamped flyback converter can beachieved. In this example, it is controlled such that the output voltageVo to a load is kept constant and that the voltage vc across thecapacitor Cr is set to a stabilized voltage. In addition, properlysetting the delay times Δt1 and Δt2 enables zero-voltage switchingoperations of Q1 and Q2, thus significantly reducing the switching loss.

Although operation as a constant-voltage power-supply unit is describedin the example described above, since Vo and vc are detected todetermine the ON-periods ton1 and ton2 of the switching elements Q1 andQ2, respectively, the voltages Vo and vc can be controlled to satisfypredetermined conditions by controlling ton1 and ton2, respectively.

The first and second switching control circuits CNT1 and CNT2 may detecta voltage at the inductor Lr generated in accordance with turning off ofQ1 and Q2.

A switching power-supply unit according to a third preferred embodimentwill be described with reference to FIG. 3. Part (A) of FIG. 3 is acircuit diagram of the switching power-supply unit, and part (B) is anillustration showing the relationship between the waveform of eachsection of the switching power-supply unit and time.

Unlike the case shown in FIG. 2, a tertiary winding Lt of thetransformer T is provided and a rectification and smoothing circuitconstituted by the rectifier diode Ds2 and the smoothing capacitor C2 isconnected to the tertiary winding Lt in this example. The secondswitching control circuit CNT2 detects the output voltage Vo2 of thesecond output terminal OUT2 and performs feedback control. The otherstructures are similar as in the second preferred embodiment. Thisswitching power-supply unit operates as a voltage-clamped flybackconverter.

Thus, the output voltages Vo1 and Vo2 can be kept at predeterminedvoltages by controlling the ON-periods ton1 and ton2 of the first andsecond switching elements Q1 and Q2 by the first and second switchingcontrol circuits CNT1 and CNT2, irrespective of the voltage of an inputpower supply vi and a load current.

A switching power-supply unit according to a fourth preferred embodimentwill be described with reference to FIG. 4. Part (A) of FIG. 4 is acircuit diagram of the switching power-supply unit, and part (B) is anillustration showing the relationship between the waveform of eachsection of the switching power-supply unit and time.

As shown in part (A) of FIG. 4, the first switching element Q1 and acapacitor Cr1 are connected to each other to define a closed looptogether with the inductor Lr and the primary winding Lp of thetransformer T. The first and second switching elements Q1 and Q2 areconnected in series with each other, and the second switching element Q2and a capacitor Cr2 are connected to each other to form another closedloop together with Lr and Lp. The rectifier diodes Ds1 and Ds2 areconnected to secondary windings Ls1 and Ls2 of the transformer T,respectively, and they define a rectification and smoothing circuittogether with the smoothing capacitor Co.

The first switching control circuit CNT 1 receives a time when thevoltage (transformer voltage) of the transformer T rises as a trigger.The first switching control circuit CNT1 also detects the output voltageVo, and controls the ON-period of the first switching element Q1 suchthat Vo is set to a predetermined voltage.

The second switching control circuit CNT2 receives a time when thetransformer voltage of the transformer T falls as a trigger. The secondswitching control circuit CNT2 also detects the transformer voltage Vtof the transformer T, and turns off Q2 when Vt becomes zero.

In part (B) of FIG. 4, Vt represents the waveform of the transformervoltage, and it represents the waveform of a current flowing in theprimary winding Lp of the transformer T. Also, Q1 and Q2 represent thestates of the first and second switching elements Q1 and Q2,respectively. Here, a high-level represents an ON-state, and a low-levelrepresents an OFF-state.

(1) State 1 [t0-t1]

As shown in part (B) of FIG. 4, after the passage of the delay time Δt1from time t0 when the transformer voltage Vt rises, the first switchingcontrol circuit CNT1 turns the gate voltage of Q1 to the high-level, andQ1 is thus turned on.

After turning on Q1, the ON-period ton1 of Q1 is determined such thatthe output voltage Vo is set to a predetermined voltage. In other words,at a time after the passage of Δt1+ton1 from time t0, the gate voltageof Q1 is turned to the low-level. Thus, Q1 is turned off.

(2) State 2 [t1-t0]

When Q1 is turned off, the transformer voltage Vt falls. After thepassage of the delay time Δt2, the second switching control circuit CNT2turns the gate voltage of Q2 to the high-level using the time when thetransformer voltage Vt falls as a trigger. Thus, the second switchingelement Q2 is turned on.

When the transformer voltage Vt becomes zero, the second switchingcontrol circuit CNT2 turns the gate voltage of Q2 to the low-level.Thus, Q2 is turned off.

When Q2 is turned off, since the transformer voltage Vt re-rises, afterthe passage of the delay time Δt1, the first switching control circuitCNT1 turns the gate voltage of the first switching element Q1 to thehigh-level using the re-rising of the transformer voltage Vt as atrigger. Time t0 here is equal to the initial time t0.

As described above, by repeating a cycle T shown in part (B) of FIG. 4as one cycle, operation as a current-resonance half-bridge converter canbe achieved.

According to this preferred embodiment, when the transformer voltage Vtbecomes zero, since the second switching element Q2 is turned off, thetransformer current (the current it flowing in the primary winding Lp ofthe transformer T) whose phase is delayed with respect to thetransformer voltage Vt allows the parasitic capacitances of Q1 and Q2 tobe charged and discharged, thus enabling a zero-voltage switchingoperation of Q1. As a result, the switching loss of Q1 and Q2 can besignificantly reduced. Although both the capacitors Cr1 and Cr2 arepreferably used in the preferred embodiment of the present inventionshown in FIG. 4, similar advantages can be achieved by eliminating oneof the capacitors Cr1 and Cr2.

A switching power-supply unit according to a fifth preferred embodimentwill be described with reference to FIG. 5. Part (A) of FIG. 5 is acircuit diagram of the switching power-supply unit, and part (B) is anillustration showing the relationship between the waveform of eachsection of the switching power-supply unit and time.

Unlike the case shown in FIG. 2, the second switching control circuitCNT2 detects a current is flowing in the secondary winding Ls of thetransformer T to determine the ON-period ton2 of the second switchingelement Q2.

In part (B) of FIG. 5, Vt represents the waveform of the transformervoltage, is represents the waveform of the current flowing in thesecondary winding Ls of the transformer T. Q1 and Q2 represent thestates of the first and second switching elements Q1 and Q2,respectively. Here, a high-level represents an ON-state, and a low-levelrepresents an OFF-state.

(1) State 1 [t0-t1]

After the current is becomes zero and the delay time Δt1 passes, thefirst switching control circuit CNT1 turns the gate voltage of the firstswitching element Q1 to the high-level, and Q1 is thus turned on. Thefirst switching control circuit CNT1 determines the ON-period ton1 of Q1such that the output voltage Vo is set to a predetermined voltage, andturns off Q1 at time t1.

(2) State 2 [t1-t0]

Thus, the transformer voltage Vt inverts, and the second switchingcontrol circuit CNT2 uses the inversion as a trigger. After the passageof Δt2, the second switching control circuit CNT2 turns the gate voltageof the second switching element Q2 to the high-level. Thus, Q2 is turnedon. When the current is of the secondary winding Ls becomes zero, thesecond switching control circuit CNT2 turns the gate voltage of Q2 tothe low-level using the current becoming zero as a trigger, and turnsoff Q2. Thus, the ON-period ton2 of Q2 is determined. This time is equalto the initial time t0 described above.

By repeating the operation described above, operation as aconstant-voltage power-supply unit can be achieved.

According to this preferred embodiment, since the second switchingelement Q2 is turned off when the secondary winding current becomeszero, the conduction period of the rectifier diode Ds and the ON-periodof Q2 are equal to each other. As a result, Q2 can be turned off whenthe current flowing in Q2 becomes zero, and a zero-current switchingoperation can be performed, thus significantly reducing the switchingloss. In addition, an effective current and the peak value of thecurrent is flowing in the switching element Q2, the rectifier diode Ds,and the transformer T are reduced, thus reducing the conduction loss.

It should be understood that the foregoing description is onlyillustrative of the present invention. Various alternatives andmodifications can be devised by those skilled in the art withoutdeparting from the present invention. Accordingly, the present inventionis intended to embrace all such alternatives, modifications, andvariances that fall within the scope of the appended claims.

1. A switching power-supply unit comprising: an inductor or atransformer; a plurality of switching elements arranged to switch acurrent flowing in the inductor or the transformer and to convert powerby turning on and off the plurality of switching elements; and aplurality of switching control circuits arranged to turn on a next oneof the plurality of switching elements in accordance with a change of avoltage or a current generated due to turning off of one of theplurality of switching elements in an ON-state, to sequentially turn onand off the plurality of switching elements in accordance with eachother, to repeat a series of on-off operations of the plurality ofswitching elements periodically, to determine an ON-period of each ofthe plurality of switching elements in accordance with a conditionindividually provided for each of the plurality of switching elements,and to control the ON-period of each of the plurality of switchingelements; wherein the plurality of switching elements includes at leastfirst, second, and third switching elements; the plurality of switchingcontrol circuits includes at least first, second, and third switchingcontrol circuits; the first switching control circuit determines anON-period of the first switching element such that a first outputvoltage is set to a predetermined value; the second switching controlcircuit determines an ON-period of the second switching element suchthat a second output voltage is set to a predetermined value; the thirdswitching control circuit determines an ON-period of the third switchingelement such that a third output voltage is set to a predeterminedvalue; and the predetermined values of the first, second, and thirdoutput voltages are different from one another.
 2. The switchingpower-supply unit according to claim 1, wherein a dead time in which twoconsecutive ones of the plurality of switching elements are turned offis provided between ON-periods of the two switching elements, andwherein the dead time is arranged in accordance with a delay time fromturning off of the switching element in the ON-state and turning on ofthe next switching element.
 3. The switching power-supply unit accordingto claim 2, wherein the dead time is set such that the switching elementis turned on when a voltage across the switching element becomes zero oris reduced to near zero.
 4. The switching power-supply unit according toclaim 1, wherein the switching control circuit turns on the next of theplurality of switching elements using a voltage at the inductor or thetransformer generated due to turning off of the one of the plurality ofswitching element in the ON-state.
 5. The switching power-supply unitaccording to claim 1, wherein the switching control circuit detects anoutput voltage to a load to determine the ON-period in accordance withthe output voltage.
 6. The switching power-supply unit according toclaim 1, wherein the switching control circuit detects a change or apolarity of a voltage generated at the inductor or the transformer todetermine the ON-period.
 7. The switching power-supply unit according toclaim 1, wherein the switching control circuit detects the currentflowing in the inductor or the transformer to determine the ON-period.8. The switching power-supply unit according to claim 1, wherein theswitching control circuit detects a voltage across the switching elementto determine the ON-period.
 9. The switching power-supply unit accordingto claim 1, wherein the switching control circuit detects a currentflowing in the switching element to determine the ON-period.
 10. Theswitching power-supply unit according to claim 9, wherein the switchingcontrol circuit determines the ON-period of the switching element suchthat the switching element is turned off when the current flowing in theswitching element becomes zero or reaches near zero.